The point of departure for the invention is represented primarily by a known RC coupling filter in the form of a bandpass filter (see DE-AS No. 24 36 966). This known RC coupling filter is a double circuit active filter which affords a number of advantages.
Although excellent results have been obtained with the known double circuit RC coupling filter, it has been considered that further improvement in the properties could be achieved through series connection of several such double circuit RC coupling filters, and in particular an improved response time in comparison to a single double circuit RC coupling filter (again see DE-AS No. 24 36 966). Application of series connection of such double circuit RC coupling filters does, of course, create the problem that the coupling factors between the individual double circuit RC coupling filters are not determined by the relationship of the coupling components substantially or objectively present. Hence the problems connected with coupling factors which are eliminated with such series connections reappear to a certain extent, since the detuning coupling factors between the individual series components are as small differences in high frequencies very sensitive to variations in the elements.
LC coupling filters are also known; among others a double circuit LC coupling filter with passive LC four terminal networks (see DE-AS No. 19 14 893). In the case of these double circuit LC coupling filters, the coupling energy is exchanged back and forth between the two circuits by way of a single coupling element, specifically, through the mutual inductance which is inevitably present. In addition, multiple circuit LC coupling filters used as high frequency bandpass filters are known. These filters consist of coupled passive two terminal networks (see Taschenbuch Electrotechnik (Handy Reference Manual for Electrical Engineering), Volume 2, Informatik (Data Processing), by E. Philippow, Hanser Verlag, Munich and Vienna, 1977, page 556). The individual LC two terminal networks in such a known LC coupling filter may be designed as parallel circuits or, after appropriate conversion, as series circuits.